Cell cultures prepared from fetal mammalian central nervous system were used to analyze developmental and functional mechanisms. The critical period for neuronal death after blockade of electrical activity was found to correspond to the time during which a major increase in basal choline acetyltransferase (CAT) activity occurred. Sodium channel density, as measured by 3H-saxitonin binding, exhibited a major increase throughout the critical period. Cyclic AMP-stimulating agents were found to induce CAT activity in spinal cord neurons. This induction was possible only while basal CAT continued to increase developmentally and it appeared to be regulated at the transcriptional level. A trophic substance shown to affect neuronal survival after electrical blockade was found to be non-dialyzable and filtrable (0.22 Mu). A procedure for obtaining this factor in serum-free media was devised. Purification of releasable factors will be greatly facilitated by this technique. Baclofen specifically reduces excitatory transmission between cultured spinal cord (SC) neurons by reducing transmitter output. It also significantly reduces calcium spike currents, but further experiments to investigate the relationship between calcium influx and transmitter output are necessary before it can be concluded that the effect of baclofen on the calcium current is sufficient to explain its synaptic action. Voltage clamp techniques reveal that the amplitude of excitatory synaptic currents varies linearly with membrane voltage, while the responses to the neurotransmitters glutamate and asparate vary non-linearly.